Traditional ammunition magazines for automatic and semiautomatic firearms, in particular, “single-stack” pistol magazines, comprise a tube with a set of feed lips flanking the opening of the tube. These feed lips retain and control the position of the first cartridge in the magazine and guide it into the barrel chamber during feeding. Typically, the feed lips are symmetric about the center of the magazine tube, and the cartridges feed from the magazine on center. One of the drawbacks of a single stack magazine tube is the direct vertical stacking of cartridges, which can position the rim of the first cartridge directly over the rebate of the second cartridge immediately below it. This can result in “rim lock”, where the rim of the first cartridge engages the rebate of the second cartridge, which can hinder cartridge feeding and even result in a malfunction. There is clearly an opportunity to improve the reliability of semiautomatic and automatic firearms by mitigating the effects of rim lock.